In electrical machining, a tool or working electrode is spacedly juxtaposed with a workpiece across a machining gap filled or flooded with a machining liquid and a machining electric current is passed between the tool electrode and the workpiece through the gap to "machine" the workpiece. In EDM, ECM and ECDM, machining signifies the removal of material from the workpiece and in electroplating or electroforming, "machining" signifies deposition of metal from the machining liquid (electrolyte) on the workpiece. In EDM, ECM and ECDM, which are called electroerosive machining in general, machining current is commonly or preferably in the form of a succession of electrical pulses. A pulsed electric current has also been recognized to be advantageous in electrodepositing. As "machining" proceeds, the tool or working electrode can be displaced (advanced or retracted) relative to the workpiece, generally by means of a servo feed system, so as to maintain the size of the machining gap substantially constant.
In electrical discharge machining, a series of timespaced, discrete electrical discharges are created across the machining gap to erosively remove material from the workpiece. Machining chips, gases and tar are thus produced, tending to accumulate in the machining gap and excessively contaminate the machining region and thereby tending to give rise to continuous arcing or short-circuiting conditions. In order to facilitate removal of these gap products from the machining zone, therefore, it has been recognized to be desirable that apart from the servo-feed system principally designed to follow up material removal and to respond to a temporary short-circuiting, means be provided designed to reciprocate the working electrode or periodically retract it with a given time interval or in response to accumulation of the discharge products in the machining gap, thereby assuring consecutive renewal of fresh or less-contaminated machining liquid in the machining site.
A typical prior EDM electrode feed system having the capability of not only servo feed but also electrode retraction and reciprocation as described is shown schematically in FIG. 1 of the attached drawing. The system includes an electric motor, e.g. a DC motor, stepping motor or AC motor, mounted on a column 2 standing upright from a bed 3 of an EDM machine equipment. The motor 1, which is rotatable bidirectionally, has its output drive shaft drivingly connected to a feed screw 4 which extends into a hollow machining head 5. The feed screw 4 is in mesh with a feed nut 6 secured to the machining head 5 at its shoulder. The machining head 5 is slidably guided on a vertical guide plane 5a in the column 2 for vertical up and down movement and is restrained from rotary movement. The machining head 5 is shown carrying a working electrode 7 juxtaposed with a workpiece 8 which is securely mounted on a work support 9 carried by a cross-feed arrangement 10 for positioning the workpiece in a horizontal plane. The workpiece 8 is immersed in a liquid dielectric 10' contained in a work tank 11. The motor 1 responds not only to a servo feed signal furnished by a servo control circuit 12 in response to a gap signal to displace the working electrode 7 so as to maintain the gap spacing, but also to a drive signal furnished by a reciprocation control circuit 13 to effect reciprocation or cyclic retraction of the working electrode 7. The servo gap signal is derived from the machining gap between the electrode 7 and the workpiece 8 and may as typical be sensed at a gap sensing circuit 14, connected to the electrode 7 and the workpiece 8. The reciprocation control circuit 13 is fed with a preset signal and may additionally be responsive to a gap signal.
It is customary that the extent of the retraction or the amplitude of the reciprocation of the electrode amounts at maximum to several millimeters in electrochemical machining and electrodeposition but is as small as at most 1 mm and typically 0.05 to 0.8 mm in electrical discharge machining. The period of intermittent retraction or reciprocation is typically less than 1 second. It is well known that the particular extent or amplitude and the period of reciprocation or intermittent retraction depend on the particular methods of electrical machining and the particular machining results required and conditions encountered and is preset in the control circuit 13.
On the other hand, it is increasingly being recognized that the servo system employing a motor 1, with a feed screw 4 and nut 6 transmission, especially designed to step-feed or incrementarily feed with a fine preset increment of displacement the machining head 5 and the tool electrode 7, is advantageous and gives rise to a marked improvement in operation efficiency over hydraulic servo systems.
Regardless of the type of motor employed, however, it has been found that motor servo systems are much slower in response than hydraulic servo systems and, when operated to effect intermittent retraction or reciprocation of the tool electrode in addition to the servo feed function, are incapable of providing sufficient machining efficiency, especially where a narrow and deep hole or recess is to be machined or a massive and heavy electrode is used.